1. Field of the Invention
The present invention relates to a ball screw groove rolling method used in manufacturing a screw shaft of a ball screw.
2. Description of the Related Art
A screw shaft of a ball screw (which is hereinafter referred to as a ball screw shaft) is generally structured in such a manner that a ball screw groove is formed in a cylindrical bar-shaped member; and, when such ball screw shaft is manufactured by rolling, in many cases, there is employed a method which is referred to as a through rolling method. This through rolling method is a method in which a ball screw groove is rolled in a cylindrical bar-shaped member using a so called walking phenomenon; and, this walking phenomenon is a phenomenon in which there is formed a slight angle difference between the lead angle of a thread rolling die and the lead angle of a ball screw groove to be rolled, and a bar member is allowed to move between the thread rolling dies in the axial direction according to the thus formed angle difference.
In case where a ball screw groove is rolled in a bar-shaped member according to such through rolling method, there can be obtained a ball screw shaft which has a length greater than the width of the thread rolling die, which makes it possible to reduce the size of a thread rolling machine which is used to manufacture a ball screw shaft.
However, in case where a ball screw groove is rolled according to the above-mentioned conventional method, in a bar-shaped member in which the ball screw groove is rolled, there occur cyclically the following error components: that is, (1) an error component in the ball screw groove due to the swinging motion of a work (a bar-shaped member); (2) an error component in the ball screw groove due to the swinging motion of a thread rolling die; and, (3) an error component in the ball screw groove due to a difference between the number of grooves of the thread rolling die and the number of grooves of the ball screw. Especially, there is a possibility that, when the ball screw groove rolled in the bar-shaped member moves out from between the thread rolling dies, stresses can be concentrated on the edge portions of the thread rolling dies and such stress concentration can crush the surface of the bar-shaped member too excessively. In case where such phenomenon occurs intermittently, the ball screw groove rolled in the bar-shaped member is made corrugated in shape; and thus, in case where the ball screw is operated at a high speed, vibrations are easy to occur and, as a result of this, there is a fear that noises can be generated and the positioning precision can be degraded. Also, there arises a possibility that the operation performance of the ball screw can be lowered and the rigidity thereof can be unstabilized.